Single Screw Pump
Single Screw Pump
What is a single screw pump?
A single screw pump is also known as a progressive cavity pump. This is a type of rotary positive displacement pump. Single screw pump manufacturers make the pump for transporting sludge and liquids. These pumps can also work on multiphase liquids that contain gas slugs common during crude oil extraction. A single-screw pump is designed to give it the ability to pump a liquid that is proportional to its speed thus giving a linear predictable rate of pumping across pressure ranges. The technology employed by single screw pump manufacturers makes these pumps deliver high pressures of around 48 bar and flow rate of 600m3 /h as well as high efficiency in the range of 55% to 75%. These pumps are most suitable for working on very viscous fluids.
Figure: Single screw pump.
How does a single-screw pump work?
The design of a single screw pump consists among other components a prime mover. The prime mover is the source of power used to run the pump. The prime mover is either internal combustion (IC) engine or an induction electric motor. The prime mover is connected to a gearbox. The gearbox is meant to reduce speed from the prime mover as single-screw pumps operate at low revolutions per minute (RPM) relative to other pumps like centrifugal pumps. Power is then transmitted from the gearbox using the output shaft. This shaft is used to connect a rotor using a universal joint to rotate the rotor in a rubber stator. The stator contains cavities. The rotor is used to push the liquid via the cavities as it rotates. The ability of the pump to produce pressure will depend on the number of pump cavities. High-pressure single screw pump designs are often made of more than one stator. A single screw pump works by employing the positive displacement principle. This pump has an inlet section that feeds fluid into a long casing. The casing is where the rotor and stator are mounted. When the rotor starts rotating it creates cavities. The cavities draw the media being transported and progress it along with the casing until it exits via the discharge port.
Working of a single screw pump.
Types of single screw pumps
Single screw pumps can be categorized on the various basis such as design, application, and orientation among others. Considering such factors there are several types of single screw pumps as follows.
Hopper single screw pump
This is a single screw pump with a hopper fitted on it. This type of single screw pump is meant for pumping very viscous liquids, media with a lot of dry matter, materials that plasticize, and large solids that need to be broken. The hopper is used to feed the media to be transported into the pump. In some cases, the length of the hopper can be customized to suit more materials or liquids according to the user’s demands.
Single screw pump with a hopper
Food grade Single screw pump
This is a model of a single screw pump designed to enhance a high level of hygiene. Single screw pump manufacturers design this pump with materials required according to EHEDG or 3A to make it easy to clean thus enhancing hygiene. Such models of single screw pumps for food applications are sometimes mounted on trolleys to make relocation easier.
food grade single screw pump (credit: Nova rotors)
Macerator single screw pump
This is a single screw pump with a strong cutter used for shredding and cutting solid particles before they enter into the pumping element. As such, the media being pumped can be transported efficiently without causing failure or blockage to the intended destination. This type of single screw pump is suitable for both commercial and domestic applications such as sewage where it can cut and shred solid particles that can clog the pump. It can also be used to shred food particles being pumped to their required point.
Figure: Macerator single screw pump (with a hopper and cutter).
Vertical immersion single screw pump
These are single screw pumps meant to be immersed in fluid for example in tanks. This helps to eliminate issues about the net positive suction head (NPSH) as they can accept NPSH of 0.5m. This makes these pumps suitable for closed or open-drain applications.
Figure: Vertical immersion single screw pump (credit: Nova rotors).
Multiphase design single screw pump
This is a single screw pump designed with a baseplate for multiphase boosting. This multiphase boosting helps the pump operator to select the pump in a way that makes it handle different fluids and solids such as oil, water, sand, and gas slugs by use of automatic remote operation.
Applications of single screw pumps
- Single screw pumps are used in dosing and metering.
- They are used in pumping wastewater such as in municipal sewer lines.
- Single screw pumps are used to pump chemicals during the manufacturing process.
- Single screw pumps are used to pump oil and petroleum during production.
- They are used in the processing of foods and beverages.
- They are used in the manufacturing of paper and pulp.
- They are used in the manufacturing of cement.
- They are used in pumping lubrication oil.
- Single screw pumps are used to pump slurry and sludge.
Advantages of single screw pumps
- Single screw pumps have low flow pulsation. This happens because these pumps have low rpm which makes the flow pulsate slowly producing low levels of acceleration head. The pumps also have stators and long pitch rotors which further reduce pulsation.
- Single screw pumps have low shear. This helps the pumps to handle any difficult media such as viscous foods, resins, water emulsions, and oils without changing the consistency of the fluid. This also makes them useful in the separation of oil and water as they ensure droplets of oil remain intact.
- Single screw pumps are reversible with decreased output pressure. As such, hoses can be cleared or if the pump is blocked it can be reversed to clear blockages. This reversible characteristic helps single screw pumps in situations such as tanker offloading and loading.
- Single screw pumps can handle a wide range of fluids. These pumps can pump viscous liquids, abrasive materials, large solids, gas slugs, fibrous solids.
- Self-priming. Single screw pump manufacturers make the pumps with tight tolerances that make them have high suction abilities to self-prime at low NPSH.
- High accuracy. These pumps have their speed proportional to flow. Due to its design of cavity, it is very easy to predict the magnitude of the flow. This makes it suitable for use in dosing and metering applications.
- They have high flow rates. Single screw pumps can achieve a flow rate of around 600m3 /hour due to their positive displacement design.
- High-pressure capabilities. Single screw pumps can produce high pressure suitable for transporting media over high heads.
- Single screw pumps operate quietly as they have low levels of vibration.
- These pumps can be installed and operated vertically.
Disadvantages of single screw pumps
- Single screw pumps need a fluid film to keep the contacting (sliding) surfaces well lubricated.
- These pumps move slowly relative to other pumps.
- These pumps can only pump fluids for short-range distances.
- Single screws have a high slippage rate emanating from stator and rotor fits reducing the pump efficiency.
- Single screw pump efficiency can be affected when the pump is working on viscous fluid if the fluid does not flow quickly into the pump. As such, speed limits for given fluids need to be ascertained to enhance the proper performance of the pump.
Factors to consider when purchasing a single-screw pump
Just like other pumps, selection of a single screw pump would be done best after considering the media to be pumped. In this case, several media characteristics need to be analyzed which are:
- Solid to liquid ratio. In essence, this is the ratio of solids to liquid in the fluid. The number of solids will affect the characteristics of the slurry.
- Nature of the liquid to be pumped. The fluid to be transported could be viscous or non-viscous, abrasive or corrosive. To confirm the nature of the fluid so you select the best and most economical single-screw pump for that application.
- Characteristics of solids in suspension. Some fluids may contain solid particles larger in size and of inconsistent shape. These could be hard, round, soft, light, dense, non-pulpy, pulpy, or fibrous. As such, it would be very important to make sure that the single screw pump selected has enough cavity size to allow the passage of solid particles of various sizes.
It is very important to first determine the minimum and maximum temperature ranges of the fluid to be pumped. This is because very high temperatures could damage some pump materials for example the stator which is made of rubber. A damaged stator would then make the pump unoperational. Also, when the temperature is very low it could affect the viscosity of that fluid thus affecting the power requirements as well as flow characteristics like flow rate.
To enhance the smooth working of the pump pressure required to transfer the media through the pipes needs to be known. This will also affect the differential pressure that is the pressure difference between the inlet and outlet sections of the pump. A pump is likely to slip if it works at increased pressure than it can handle. Pump slip is when a fluid pump losses its pumping capacity whereby fluid starts returning from the discharge side to the inlet. If the pressure required is high then an additional stator or rotor can be supplemented. Also, the pressure affects the wear of the pump in that at high differential pressure there is high abrasive wear.
The viscosity of the fluid being pumped affects the working temperature. In some cases, some fluids are very viscous and thus they are heated to reduce their viscosity. Viscosity reduces with an increase in temperature and vice versa.
Abrasion is one of the main reasons your single screw pump can fail. The characteristics of the abrasion wear depend on the nature of the solid particles to be pumped. There are various types of abrasives which can help you to select the right single screw pump. They are:
- Light abrasives. This refers to solid particles in a fluid such as a slit, soil, or dirt.
- No abrasives. This refers to a case whereby the fluid being pumped does not have solid particles such as clear water, gasoline, lubricating oil, and gasoline.
- Medium abrasives. This is whereby the fluid being pumped has solids such as clay slurries, wood dust, sludge, frit, and porcelain enamel.
- Heavy abrasives. This refers to solid particles loaded with sand, emery dust, lapping compounds, mill scale, and grout and roof gypsum.
Installation and environmental considerations
Single screw pumps are used in a wide range of environmental conditions. As such, it is important to put into consideration the environment and installation of the pump. This includes considerations such as hazardous environment among other factors like portability, reversible flow, and submersible conditions among others. The space required and orientation of installation is also important to consider.
Troubleshooting single screw pump
The pump is not starting
- There is adhesion on the rotor and stator. Lubricate stator and rotor using inert oil or glycerin.
- Excess outlet pressure. Increase motor or pump size. Change pipe configuration or system set up to lower outlet pressure.
- Remove solids in inspection ports. If it persists dismantle your pump so that you can easily clean it.
- Fluid temperature. Check fluid temperature to ascertain it is as required by the manufacturer.
- Large solid particles. Install filter at the suction side or reduce the speed of the pump.
- Crystallization/sedimentation. Flush the pump or dismantle it so that it can be cleaned properly.
- Frozen fluid. Install trace heating or protect the pump from elements and drain it when not in use.
- Stator not compatible with fluid. Check stator and fluid chemical compatibility.
- Incorrect speed of the pump. Check pump speed. Ensure the rpm, voltage, frequency, and power absorbed areas required.
- Too high viscosity. Check against pump manufacturer specifications.
Incorrect flow rate
- Excessive outlet pressure. Increase motor size or pump size. Change pipe configuration or system set up to lower outlet pressure.
- Air ingress via inlet pipework or seal. Replace seal and tighten joint bolts.
- Remove solids from the inspection port. Dismantle pump for effective cleaning.
- Worn stator. Replace stator.
- Worn rotor. Replace rotor.
- Faulty or broken transmission. Repair broken or faulty transmission.
- Excessive pressure on the outlet. Use a larger motor or pump. Change piping or system configuration.
- Too high suction speed. Check inlet filter, check inlet pressure, and suction line is opened fully.
Excess pump vibration
- Worn out bearings. Replace bearings.
- Large solid size. Install filter at the inlet or reduce the speed of the pump.
Single screw pumps also known as progressive cavity pumps are rotary positive displacement pumps. This pump work by using power from an electric motor or IC engine. The prime mover transmits its power to the gearbox via a shaft. The gearbox reduces the speed as single screw pumps work at low rpm. The power from the gearbox is then transmitted to a rotor and stator. As the rotor rotates it pushes fluid through cavities on the stator.
Single screw pumps are used in many industries among them paper and pulp, cement and clay, ship building, agriculture, petrochemical, sewage treatment among others. Single screw pumps are used in many industries due to their superb properties such as producing constant and non-pulsating flow, reversible, less noisy, ability to handle highly viscous fluids among others.
There are several types of single-screw pumps which are named depending on their functions such as food-grade single screw pump, macerator single screw pump, vertical immersion single screw pump, and multiphase single screw pump. Before purchasing a single screw pump, one must consider technical factors such as type of media to be pumped, temperature viscosity, pressure, abrasion, environmental and installation conditions.